This invention relates to air cooling systems for gas turbine engines.
It is customary to bleed compressed air from one or more of the compressors of a gas turbine engine to provide cooling of various static and rotating components.
If air is used to cool high speed rotating components the friction caused by the air on the rotating components can give rise to appreciable heating of the air. This form of heating of the air is often referred to as "windage heating". It is known from British Pat. No. 1,217,807 that "windage heating" may be reduced by preswirling the air in the same direction as the rotating components. Ideally the volume of swirling air has a swirl velocity the same as the angular velocity of the rotating components.
In many designs of gas turbine engine the swirling air is caused to flow through narrow passages defined by a static structure and the rotating component to prevent or reduce recirculation of the cooling air to regions of the engine where it would be preferable not to flow the cooling air.
There are many advantages in being able to keep the cooling air as cool as possible and to avoid windage heating. For example, one can achieve lower temperature environments thus enabling present materials to be used with a greater safety margin or to enable cheaper materials to be used.
However, it has been found that the viscous drag of the swirling air on static structures and walls can slow the swirling air to such an extent that the swirl velocity is at variance with the angular velocity of the rotating components and gives rise to windage heating of the air.